Surface roughness is a term used to describe the physical profile of a surface and may be taken to include random surface deformations as well as periodic deformations. Surface corrugations is a term used to describe periodic deformation. Corrugations have been a major source of concern to the railroad and transit industries. In railroads and transit operations, such corrugations appear around curves of the railroad track and in areas of repeated or consistent application of heavy breaking. They result from vibration induced slippage of one or more of the wheels in the wheel set. Such slippage tends to take place at the same spots so that the amplitude of the corrugations become larger quite rapidly after they have begun. Such corrugations are undesirable since the vibrations caused by the corrugated rail tends to loosen rail fastenings, displace ballast and damage the rolling stock and cargo. Additionally, such corrugations tend to accelerate deterioration of the track roadbed, not to mention the general deterioration of the rail itself. From the passenger point of view, corrugations are noticeable in producing unwanted vibrations and excessive roaring noise. The corrugation characteristics depend upon the mechanical characteristics of the train and the elasticity of the railroad track, the speed of the train and the train speed in relation to the track curvature. Periodic maintenance work is scheduled to correct rail corrugations based on either historical data or measurements of the rail corrugations. Measurements of the rail corrugations are typically done both before and after rail maintenance procedures to ensure correction of the undesirable deformations at each of the wavelengths which are measured. Such wavelengths of interest are in the range of 3 centimeters to 3 meters. However, typically many different wavelengths are superimposed along a section of the track thus complicating detection techniques. Railroad maintenance may include, among other things, the use of grinding wheels or abrasive blocks moved along the rail surface.
There are basically two types of measurement svstems employed in corrugation detection; namely, inertial and non-inertial systems. Inertial systems utilize inertial sensors to provide an indirect surface measurement. Such sensors operate by detecting an acceleration, either translational or rotational, between the rail surface and the supporting chassis. Non-inertial systems utilize noninertial sensors which operate to directly measure the distance between the supporting chassis and the rail. Such non-inertial sensors may be classified as belonging to the contacting or non-contacting type. The contacting type of detector may simply be in the form of an auxiliary wheel or feeler in which the vertical displacement of the wheel is detected by electromechanical means. Non-contacting sensors may be of the eddy current type, acoustical or electromagnetic. An example of a non-inertial data measuring and processing system is shown in U.S. Pat. No. 4,288,855.
Corrugation, and more generally, surface roughness problems are not isolated to the railroad industry. Highway roughness, including corrugations, are a major contributor to highway wear, particularly at intersections where the road surface is subjected to large stresses from acceleration and deceleratio of passing vehicles. In pouring concrete for forming highway surfaces, roughness is an important characteristic which affects the quality of the finished product and its long-term wear characteristics. In such an environment, a non-contacting type sensor is important since the measuring equipment cannot itself contribute to surface roughness.
Surface roughness problems are also noticeable in other technologies such as pipe and drum fabrication or more generally in the measurement of any surface contour. The surfaces of gears, cams and many types of precision equipment require accurate measurements of surface profile.
A particular problem inherent in the utilization of inertial sensors for roughness and corrugation measurements, for example rail corrugation measurements, is the fact that such sensors are subject to small signal to noise ratios at low vehicle speeds. Concomittantly, the integration time must be greatly increased in order to permit translation of the acceleration data into profile information of a given wavelength. With such long integration times, the data suffers from even lower signal to noise ratios. Non-inertial sensors of the contact type suffer from the influence of chassis vibrations caused, for example, by wheel roughness and inherent chassis elasticity. Non-contact sensors do not suffer from the same vibrational problems as sensors of the contact type but generally do require a larger number of detecting units which results in proportionally larger maintenance problems. A problem which is inherent in both inertial and non-inertial detectors is related to the plural frequency components of the corrugations. Typically, the sensor response contains nulls at different wavelengths. Methods have been devised to try and effectively remove such nulls by utilizing different measuring bases of different lengths so that the wavelenghts which are susceptible to poor response will not overlap in the simultaneous measurement. An example of such a teaching is shown in U.S. Pat. No. 4,075,888.